When exposed to radiation such as X-rays, an energy-storing phosphor (stimulable phosphor, which gives stimulated emission off) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor is widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the panel with a stimulating light such as a laser beam to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) used in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective film is ordinarily provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
Phosphor layers of various kinds are known. Examples of the known phosphor layers include a phosphor layer comprising a binder and an energy-storing phosphor dispersed therein, a phosphor layer which is formed by a gas phase-accumulation method or by a firing method and which comprises agglomerate of an energy-storing phosphor without binder, and a phosphor layer comprising energy-storing phosphor agglomerate impregnated with a polymer material.
Referential Patent Publication 1 discloses a variation of the radiation image recording and reproducing method. While an energy-storing phosphor of the storage panel used in the conventional type plays both roles of radiation-absorbing function and energy-storing function, those two functions are separated in the disclosed method. In the method, a radiation image storage panel comprising at least an energy-storing phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor (radiation-absorbing phosphor) which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of: causing the radiation-absorbing phosphor of the screen or the panel to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storing phosphor of the panel to store the energy of the converted light as radiation image information; sequentially scanning the panel with a stimulating light to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric image signals. The present invention can be also applied to the radiation image storage panel used in the method of this type.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and further give a reproduced radiation image of high quality (in regard of sharpness and graininess).
In order to improve the sensitivity and image quality, it is proposed that the phosphor layer of the storage panel be prepared by a gas phase-accumulation method such as vacuum vapor deposition or sputtering. The process of vacuum vapor deposition, for example, comprises the steps of: heating to vaporize an evaporation source comprising a phosphor or materials thereof by means of a resistance heater or an electron beam, and depositing and accumulating the vapor on a substrate such as a metal sheet to form a layer of the phosphor in the form of columnar crystals.
The phosphor layer formed by the gas phase-accumulation method contains no binder and consists of the phosphor only, and there are cracks in the columnar crystals of the phosphor. Because of the presence of cracks, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too.
Accordingly, a radiation image storage panel having the phosphor layer formed by the gas phase-accumulation method has high sensitivity. At the same time, since the cracks prevent the stimulating light from diffusing parallel to the layer, the storage panel can give a reproduced image of high sharpness.
When subjected to X-ray diffraction (XRD) measurement, crystalline material generally shows a diffraction pattern comprising diffraction lines. Widths of the lines are known to depend upon crystallite size, crystallinity, superlattice and inner distortion of the crystal (including variation of the composition) [Referential Non-Patent Publication 1]. If the degree of crystallinity (ratio of crystal contained in the material) and the superlattice (atomic rearrangement from an irregular lattice into a regular lattice) are negligible, the line width of XRD varies according to the crystallite size and the inner distortion of crystal. In that case, the relation among them can be expressed by Hall's equation (1):β cos θ/0.9λ=1/D+2ε sin θ/0.9λ  (1)[in which β is a half width specific to the sample material, D is a crystallite size, ε is a lattice distortion, θ is a Bragg angle, and λ is a wavelength of X-ray].
If β cos θ/0.9λ is plotted against sin θ/λ according to the Hall's equation (1), a straight line having the gradient of 2ε/0.9 and the intercept of 1/D is obtained. Thus obtained line can make it clear whether change (increase) of XRD line width (half width) is attributed to the lattice distortion ε or the crystallite size D.
The lattice distortion (often referred to as “lattice mismatch”) is represented by an average ratio of lattice constant changes based on the normal lattice constants. Generally, the lattice distortion is induced by inner distortion of the crystal, and the inner distortion is mainly caused by residual stress, which is caused by crushing and/or rolling of the material, or by what is called “variation of composition”, which means non-uniformness of composition given, for example, when impurities are mixed into the sample material to form a solid solution.
Referential Patent Publication 2 discloses a radiation image storage panel having a stimulable phosphor layer comprising regularly arrayed crystals. In that phosphor layer, lattice planes of the crystals are arranged so that the ratio I2/I1 may be 0.3 or less. Here, I1 and I2 are intensities of the first and second peaks, respectively, in an X-ray diffraction pattern obtained by applying X-rays in an incident angle of 10° to 35° onto the lattice plane perpendicular to the direction in which the crystal grows the fastest.
Referential Patent Publication 3 discloses a binder-less storage phosphor screen comprising an alkali metal storage phosphor. The phosphor gives an XDR spectrum in which (100) and (110) diffraction lines have intensities I100 and I110 respectively, satisfying the condition of I100/I110≧1.    Referential Patent Publication 1: Japanese Patent Provisional Publication No. 2001-255610,    Referential Patent Publication 2: Japanese Patent Publication No. 3,130,632,    Referential Patent Publication 3: Japanese Patent Provisional Publication No. 2001-249198, and    Referential Non-Patent Publication 1: Yoshinori Sasaki, et al., Kihon-Kagaku Sirizu 12 (series of basic chemistry 12), “KESSHOU-KAGAKU-NYUMON (introduction of crystal chemistry)”, 1999, pp. 71-75.